1. Field of the Invention
The present invention relates to a novel phenylacetylene compound having an alkyl group in its core, which is useful as a component of a liquid crystal display element or a component of a liquid crystal composition, a liquid crystal composition including the same, and a liquid crystal element using the same. More particularly, it relates to a liquid crystal compound having a large anisotropy of refractive index (.DELTA.n), a liquid crystal composition, and liquid crystal elements using the same, such as an optical shutter and display elements represented by a STN (supertwisted nematic) liquid crystal element, and a PDLC (polymer dispersed liquid crystal) type liquid crystal element.
2. Description of the Related Art
With the progress of information-oriented society in recent years, various kinds of display elements have more and more increased in importance as one of the man-machine interfaces. In such circumstances, a flat-panel display, especially a liquid crystal display (LCD), has, rapidly achieved widespread use because it has characteristics such as thinness, light weight, driving at a low voltage, and low consumption of electric power. Among the liquid crystal elements represented by a liquid crystal display, a matrix type liquid crystal display, which stores a large amount of information, has two driving systems referred to as an active matrix system and a passive matrix system, respectively.
In the active matrix system, a thin film transistor such as a polysilicon or amorphous silicon or a diode is provided on each pixel as a non-linear element. However, the active matrix system has some problems in increasing the picture area, lowering the price and increasing the density, because of complicated production processes and low yield. Taking price, productivity, and the like into consideration, the passive matrix system is more predominant.
As the passive matrix system liquid crystal elements which are practically used at present, TN (twisted nematic) and STN liquid crystal elements are mainly used. The TN type has found widespread application as display elements such as watches and portable calculators. With this system, rise of electrooptical properties is slow, and a contrast is considerably decreased with an increase in duty ratio, and hence it is difficult in principle to set up a display having a large picture area. The STN type is a system developed for compensating for the drawbacks of the TN liquid crystal element. It has sharply rising electrooptical properties, which enables the implementation of a large picture area. At present, it is used as a display for notebook personal computer, and the like.
However, while the STN liquid crystal element has more excellent characteristics as compared with the TN liquid crystal element, it still has some problems to be solved for a further increase in picture area, decrease in price, and increase in density.
For example, in comparison with the TFT liquid crystal element which is one of the typical examples of the active matrix system, the STN liquid crystal element is still insufficient in terms of a viewing angle characteristic and response speed. Especially, achievement of rapid response is essential for a further increase in picture area and density, displaying motion pictures, or the like.
To achieve the rapid response of the STN liquid crystal element, a decrease in cell thickness is one of effective methods. The STN type utilizes a birefringence effect for a displaying method. To use this system, it is necessary to suppress the change in tonality and optical characteristics of a panel, that is, to set a retardation at a constant optimum value. The retardation R is represented by R=(.DELTA.n.times.d), and hence the anisotropy of refractive index (.DELTA.n) is required to be increased for reducing the cell thickness d.
As a liquid crystal having a relatively large anisotropy of refractive index, a tolan compound is known [Mol. Cryst. Liq. Cryst., vol., 23, p. 233 (1973)]. However, the anisotropy of refractive index is approximately 0.2, which is not a passable value. There has also been developed a compound (2) represented by the following formula (JP-A-2-83340); ##STR2## wherein Alkyl represents an alkyl group.
The compound (R1) has an anisotropy value of refractive index of 0.3 or more. However, it has a bad compatibility with other liquid crystals, and hence it is impractical. Then, there has been developed a compound (R2) represented by the following formula for a purpose of improving the compatibility with other liquid crystals (JP-A-9-216841); ##STR3## wherein R.sup.8 represents an alkyl group, Y represents R.sup.8, a fluorine atom, a chlorine atom, a bromine atom, an iodine atom or a cyano group, H.sup.1 to H.sup.12 represent a hydrogen atom, a fluorine atom or a chlorine atom (provided that at least one of the H.sup.1 to H.sup.12 is a fluorine atom or a chlorine atom).
The compound (R2) is more improved in terms of compatibility with other liquid crystals than in the case of the compound (R1). However, the hydrogen atom has been substituted by a halogen atom such as a fluorine atom, and hence the anisotropy of refractive index is reduced. Thus, the anisotropy of refractive index is sacrificed in return for the improvement in compatibility.